Non-stick masking fixtures and methods of preparing same

ABSTRACT

Non-stick fixtures for selectively masking portions of a workpiece during application of a workpiece coating are described herein. These fixtures have predetermined surfaces thereon having an average surface roughness of about 25 Ra or less and a Rockwell hardness of about 65 Rc or more. The controlled average surface roughness ensures that these fixtures are non-stick with respect to the workpiece coating being applied to the workpieces disposed therein. The controlled Rockwell hardness ensures that the desired average surface roughness can be maintained throughout repeated use of the fixture in harsh coating environments. These fixtures reduce the workpiece coating bridging that occurs between the fixture and the workpiece, and also reduce the amount of overspray that occurs on the workpiece, thereby minimizing the amount of handwork and/or rework that is necessary after the workpiece is coated. This improves process cycle times and yields significantly.

FIELD OF THE INVENTION

The present invention relates generally to masking fixtures. Morespecifically, the present invention relates to non-stick fixtures thatare useful for masking selected portions of a workpiece duringapplication of a workpiece coating thereon, and methods of preparingfixtures for such purposes.

BACKGROUND OF THE INVENTION

Gas turbine engines have long been used to convert chemical potentialenergy, in the form of fuel, to thermal energy, and then to mechanicalenergy for use in propelling aircraft, generating electric power,pumping fluids, etc. The efficiency of gas turbine engines increaseswith increasing operating temperatures. Therefore, there is a largeincentive to raise the combustion and exhaust gas temperatures of suchengines. However, the metallic materials currently used in thehot-section components of such engines operate at temperatures very nearthe upper limits of their thermal stability. In fact, in the hottestsections of modern gas turbine engines, metallic materials are utilizedin hot gas paths at temperatures above their melting points. Thesemetallic materials survive such temperatures only because they are aircooled, or because they comprise ceramic coatings thereon that lower thethermal conductivity of the component, thereby allowing the componentsto be operated at higher temperatures while utilizing less cooling air.Such ceramic coatings may additionally or alternatively provideenvironmental protection to the metallic components, protecting thecomponents against the oxidative and corrosive effects of the hot gasespassing therethrough or thereby.

During engine operation, the airfoil and inner buttress surfaces of theturbine vanes are directly exposed to the hot gases, and are thereforesusceptible to accelerated oxidation and corrosion. Accordingly, aceramic coating is generally applied to the airfoil and inner buttresssurfaces of these vanes. In contrast, the remaining surfaces of thesevanes are not directly exposed to the hot gases, and coating theseremaining surfaces may actually be detrimental and degrade the fatiguelife of the attachment mechanisms and other highly stressed regionsthereof. Therefore, these remaining surfaces are typically leftuncoated.

Fixtures are often used to mask the selected portions of the vane thatare not being coated. However, with such fixtures, bridging of theceramic coating being applied to the vane often occurs between the vaneand the fixture itself. This bridging may cause some of the ceramiccoating to chip away from the vane, or cause extra ceramic coating toadhere to the vane, when the vane is removed from the fixture. Bothcases require a significant amount of handwork and/or rework in order toprepare the vane for use in a gas turbine engine.

It would be desirable to have fixtures for selectively masking turbinevanes or other workpieces prior to coating unmasked portions thereof. Itwould be further desirable to have fixtures that allowed such workpiecesto be coated without bridging of the workpiece coating occurring betweenthe fixture and the workpiece during application of a workpiece coating.It would be particularly desirable to have such fixtures be non-stickwith respect to the workpiece coating being deposited on the workpieceso that less handwork and/or rework than currently required would benecessary.

SUMMARY OF THE INVENTION

Accordingly, the above-identified shortcomings of existing maskingfixtures are overcome by embodiments of the present invention, whichrelates to masking fixtures that are non-stick with respect to theworkpiece coating being deposited on a workpiece disposed therein sothat workpiece coating bridging is less likely to occur between thefixture and the workpiece, thereby minimizing the amount of handworkand/or rework that is necessary after the workpiece is coated. Thesefixtures utilize controlled average surface roughnesses to ensure thatthe workpiece coating does not adhere to the fixtures, and controlledRockwell hardnesses to ensure that the fixtures can maintain thosedesired surface roughnesses throughout time in the harsh, abrasivecoating environments in which they are used.

Embodiments of this invention comprise fixtures for selectively maskinga workpiece disposed therein, the fixture being hard enough to maintainan average surface roughness of about 25 Ra or less on predeterminedsurfaces of the fixture throughout repeated applications of a workpiececoating on different workpieces disposed therein.

Embodiments of this invention also comprise fixtures for selectivelymasking a workpiece disposed therein, wherein predetermined surfaces ofthe fixture have an average surface roughness of about 25 Ra or less anda Rockwell hardness of about 65 Rc or more.

Embodiments of this invention also comprise methods for preparing afixture for selectively masking a workpiece during application of aworkpiece coating. These methods comprise the steps of: preparingpredetermined surfaces of the fixture to an average surface roughness ofabout 25 Ra or less; and ensuring that the predetermined surfaces arehard enough to maintain the average surface roughness throughoutrepeated applications of workpiece coating. The preparing step maycomprise polishing the predetermined surfaces. The ensuring step maycomprise: (a) making at least the predetermined surfaces of the fixturefrom a material having a Rockwell hardness of at least about 65 Rc; (b)coating at least the predetermined surfaces of the fixture with afixture coating to harden at least the predetermined surfaces of thefixture to a Rockwell hardness of at least about 65 Rc; and/or (c)treating at least the predetermined surfaces of the fixture to harden atleast the predetermined surfaces of the fixture to a Rockwell hardnessof at least about 65 Rc. The coating step may comprise applying thefixture coating via physical vapor deposition, chemical vapordeposition, or any other suitable fixture coating process. The treatingstep may comprise hardening at least the predetermined surfaces of thefixture via a nitriding process, a plating process, or any othersuitable surface treatment process. The fixture coating may compriseTiN, TiAlN, or any other suitable material.

The predetermined surfaces may comprise at least a portion of anyfixture surfaces where bridging of workpiece coating may occur betweenthe fixture and the workpiece. The predetermined surfaces may compriseat least any fixture surfaces that will be exposed to workpiece coatingwhen the workpiece is being coated. These fixtures should be capable ofbeing used in a thermal spray process. These fixtures may be made of A2tool steel at least partially coated with TiN or TiAlN. The workpiecemay comprise a gas turbine engine component.

Further features, aspects and advantages of the present invention willbe readily apparent to those skilled in the art during the course of thefollowing description, wherein references are made to the accompanyingfigures which illustrate some preferred forms of the present invention,and wherein like characters of reference designate like parts throughoutthe drawings.

DESCRIPTION OF THE DRAWINGS

The systems and methods of the present invention are described hereinbelow with reference to various figures, in which:

FIG. 1 is a schematic diagram showing a turbine vane, as coated in anexemplary masking fixture of this invention; and

FIG. 2 is an exploded view showing the turbine vane of FIG. 1 partiallyenclosed in an exemplary masking fixture of this invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of promoting an understanding of the invention,reference will now be made to some preferred embodiments of thisinvention as illustrated in FIGS. 1-2 and specific language used todescribe the same. The terminology used herein is for the purpose ofdescription, not limitation. Specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as abasis for the claims as a representative basis for teaching one skilledin the art to variously employ the present invention. Any modificationsor variations in the depicted structures and methods, and such furtherapplications of the principles of the invention as illustrated herein,as would normally occur to one skilled in the art, are considered to bewithin the spirit and scope of this invention.

This invention relates to fixtures that are useful for masking selectedportions of a workpiece during application of a workpiece coatingthereon, and methods of preparing fixtures for such purposes. Thesefixtures utilize controlled average surface roughnesses to make thefixtures non-stick with respect to the workpiece coatings being appliedto workpieces disposed therein, making it less likely that bridging ofthe workpiece coating will occur between the fixture and the workpiece,and minimizing the amount of handwork and/or rework that is requiredafter the workpiece is coated. These fixtures also utilize controlledRockwell hardnesses to ensure that the fixtures can maintain the desiredsurface roughnesses in the harsh coating environments in which they areused.

Referring now to FIG. 1, there is shown a schematic diagram of a turbinevane 10, as coated in an exemplary non-stick masking fixture of thisinvention. This exemplary turbine vane 10 comprises an airfoil 12, aninner buttress 20 having an inner surface 22 and a mate face 24, and anouter buttress 30 having an inner surface 32 and a mate face 34.

Typically, only the airfoil 12 and the inner surfaces 22, 32 are coated(i.e., with 7YSZ), while the other surfaces of the turbine vane remainuncoated. The portions of the vane that do not need coating (i.e., matefaces 24, 34, etc.) are generally selectively masked before being placedin a coating apparatus. This may be accomplished by using a maskingfixture. As previously noted, existing masking fixtures are less thanoptimal. However, the fixtures of this invention provide a costeffective, labor saving means for masking selected portions of a turbinevane during coating thereof. The nonstick masking fixtures of thisinvention significantly reduce the time it takes to process a turbinevane by reducing the amount of handwork and/or rework that is necessaryto prepare the vanes for use in a gas turbine engine.

The non-stick masking fixtures of this invention may comprise anysuitable shape or format, so long as the fixture adequately preventsselected portions of the vane 10 or other workpiece disposed thereinfrom being coated. In one exemplary embodiment, the fixture may comprisetwo parts: a top fixture 50 and a bottom fixture 60, as shown in FIG. 2.

The top fixture 50 may comprise a conventional, metallic coating boxhaving an end plate 51 and four walls 52 extending therefrom to form afive-sided enclosure with a partially open end 53 opposite the end plate51. The partially open end 53 may comprise an aperture therein designedto fit substantially flush at its outer surface with the inner surface22 of the inner buttress 20. Once the inner buttress 20 of the vane 10is inserted into the top fixture 50, the top fixture 50 may be securedin place in any suitable manner, such as, for example, via a set screw58.

The bottom fixture 60 may also comprise a conventional, metallic coatingbox having an end plate 61 and four walls 62 extending therefrom to forma five-sided enclosure with a partially open end 63 opposite the endplate 61. The partially open end 63 may comprise an aperture thereindesigned to fit substantially flush at its outer surface with the innersurface 32 of the outer buttress 30. In embodiments, the bottom fixture60 may comprise a removable portion 64 that can be removed from thebottom fixture 60 to enable easier insertion of the outer buttress 30 ofthe vane 10 into the bottom fixture 60. Once the outer buttress 30 ofthe vane 10 is inserted into the bottom fixture 60, the removableportion 64 can be replaced and secured in place in any suitable manner,such as, for example, via a nut and bolt 65 and an anchoring device 66.

In embodiments, predetermined surfaces on the fixtures of this inventionhave an average surface roughness of about 25 Ra or less and a Rockwellhardness of at least about 65-85 Rc or more. The relatively smoothsurfaces minimize the amount of workpiece coating that will adhere toand build up on the fixture during coating of the workpiece disposedtherein, and the hardness ensures that the desired surfaceroughness/smoothness can be maintained throughout repeated use of thefixture in a workpiece coating environment. Since these fixtures arenon-stick with respect to the workpiece coating, they prevent, or atleast minimize, bridging of the workpiece coating between the fixtureand the workpiece.

These predetermined surfaces comprise at least portions of the surfaceswhere bridging of the workpiece coating may occur between the fixtureand the workpiece. In the embodiment shown, that includes at least thoseportions of the partially open ends 53, 63 that are closest to the vane10 disposed therein. Controlling the average surface roughness andRockwell hardness of these predetermined surfaces makes it less likelythat bridging of the workpiece coating will occur between the fixtureand the workpiece disposed therein. Some or all of the other exteriorsurfaces (i.e., those surfaces that may be exposed to workpiece coatingwhen the workpiece is being coated—walls 52, 62, end plates 51, 61,etc.) may also have their average surface roughnesses and Rockwellhardnesses controlled to make more of the fixture non-stick with respectto the workpiece coating. Even the interior surfaces of the fixturescould have their surface roughnesses and hardnesses controlled, ifdesired. However, it is most important to control the surfaces wherebridging of the workpiece coating may occur.

In some embodiments, only the exterior surfaces of the partially openends 53, 63 have their surface roughnesses and hardnesses controlled. Inother embodiments, the exterior surfaces of the partially open ends 53,63, as well as the exterior surfaces of the walls 52, 62 have theirsurface roughnesses and hardnesses controlled. In yet other embodiments,all exterior surfaces of the fixture that may be exposed to workpiececoating have their surface roughnesses and hardnesses controlled. Manyother embodiments are also possible, and all are within the spirit andscope of this invention.

The predetermined surfaces of the fixture can be prepared to an averagesurface roughness of about 25 Ra or less in various ways, such as bymanually polishing the surfaces with an abrasive wheel, flap wheel orsandpaper, or in any other suitable manner.

Once the desired average surface roughness is achieved, a Rockwellhardness of about 65-85 Rc or more on at least the predeterminedsurfaces can be achieved in various ways. If the predetermined surfacesof the fixture have a Rockwell hardness of about 65 Rc, they will workwell, they just won't likely last as long as surfaces with a Rockwellhardness of closer to 80-85 Rc. In embodiments, the desired hardness canbe achieved by applying a fixture coating to the predetermined surfacesof the fixture. In other embodiments, the desired hardness can beachieved by treating the predetermined surfaces of the fixture. In yetother embodiments, the desired hardness can be achieved by making thefixture out of a material that has a Rockwell hardness of about 65 Rc ormore, so that no fixture coating or surface treatment is necessary toharden the fixture surfaces. As previously noted, controlling thehardness of at least the predetermined surfaces of the fixture ensuresthat the desired surface roughnesses can be maintained throughoutrepeated use of the fixtures, thereby maintaining the non-stickproperties of the fixture.

A fixture coating may be applied to at least the predetermined surfacesof the fixture to harden those surfaces to a Rockwell hardness of about65-85 Rc or more. When using fixture coating, the fixture itself may bemade of any suitable material that is hard enough to provide structuralsupport to the fixture coating so that the fixture coating will not besusceptible to cracking, chipping, or other premature failures. Inembodiments, these fixtures may be made of a hardened tool steel such asA2 tool steel. Preferably, the uncoated fixture material will have aminimum Rockwell hardness of about 45 Rc. The fixture coating maycomprise any suitable material that will harden the fixture surfacesenough to maintain the desired surface roughness thereon. Inembodiments, the fixture coating comprises TiN or TiAlN. The fixturecoating may be of any suitable thickness, and in embodiments, is about3-5 microns thick. This fixture coating may be applied in any suitablemanner, such as, for example, physical vapor deposition (PVD) orchemical vapor deposition (CVD).

The hardness of the predetermined surfaces on the fixture, and thehardness of the underlying material that the fixture itself is madefrom, are both important. When applying a fixture coating, it is best toapply it to the hardest possible surface to maximize the life of thefixture.

Surface treatments may also be used to harden at least the predeterminedsurfaces of the fixture to a Rockwell hardness of about 65 Rc or more.When using surface treatments, the fixture itself may be made of anysuitable material that is hard enough to be hardened to a Rockwellhardness of about 65 Rc or more with surface treatments. In embodiments,these fixtures may be made of a hardened tool steel such as A2 toolsteel, or any other material that has a minimum Rockwell hardness ofabout 45 Rc. Such surface treatments may comprise nitriding and/orplating.

To use the fixtures of this invention, an uncoated workpiece is mountedin the fixture so that selected portions of the workpiece are masked bythe fixture. When the workpiece coating is applied, the portion of theworkpiece within the fixture will remain substantially uncoated.

Existing fixtures often require large gaps (sometimes as wide as about0.060 inches) to be maintained between the fixture and the workpiecedisposed therein so that bridging of the workpiece coating is lesslikely to occur. These large gaps, however, allow significant amounts ofoverspray to accumulate on portions of the workpiece that are to remainuncoated (i.e., on mate surfaces 24, 34). Some eliminate this oversprayby taping these uncoated surfaces of the workpiece with high temperaturetape prior to inserting the workpiece into the fixture. Others eliminatethis overspray by handworking (i.e., via an abrasive wheel, handgrinder, etc.) the workpieces to remove the overspray from the coatedworkpieces as necessary. However, such handworking often creates furtherchipping and blending problems, which then requires further reworking(i.e., stripping the workpiece coating from the workpiece and thenrecoating the workpiece). Both these fixes to the overspray problem aremanual labor intensive, and increase cycle time for the workpieces.

In contrast to existing fixtures, the fixtures of this invention allowmuch smaller gaps (about 0.020 inches) to be maintained between thefixture and the workpiece disposed therein. This prevents orsubstantially minimizes the overspray that occurs on the workpiecesbeing coated, thereby significantly reducing the amount of handworkand/or rework that must be done to a coated workpiece before it can beused for its intended purpose. This also improves the yields, increasesthe throughput, and decreases the cycle time for workpieces that arecoated in the fixtures of this invention.

This invention also comprises methods for preparing fixtures for use inany suitable workpiece coating process, such as, for example, thermalspray processes such as air plasma spray (APS). First, any surfaces thatmay allow bridging of the workpiece coating to occur between the fixtureand the workpiece disposed therein should be prepared to an averagesurface roughness of about 25 Ra or less. Additional fixture surfaces,or even all the fixture surfaces, may be prepared to this averagesurface roughness, if desired. Thereafter, the critical surfaces of thefixture should be hardened to a Rockwell hardness of at least about 65Rc, more preferably about 80-85 Rc. This hardness may be established onas many of the fixture surfaces as desired, and may even include allfixture surfaces. The critical surfaces include at least portions of anysurfaces that may allow bridging of the workpiece coating to occurbetween the fixture and the workpiece disposed therein. Preparing andhardening the surfaces in this manner allows the desired surfaceroughness to be maintained in the harsh workpiece coating environmentsin which the fixtures are used.

Several sample masking fixtures were created and tested in a thermalspray environment. These fixtures were made of 340 stainless steel in afirst embodiment, and A2 tool steel in a second embodiment.Predetermined surfaces of the fixtures (i.e., the partially open ends53, 63 and portions of the walls 52, 62) were prepared to an averagesurface roughness of about 25 Ra or less by buffing and polishing.Thereafter, a fixture coating comprising TiN was deposited on the 340stainless steel and the A2 tool steel via PVD in vacuum at about 850° F.for about 1.5 hours. These fixture coatings were about 3-5 micronsthick. After the fixture coating was applied, the predetermined surfaceson both of these fixtures had a Rockwell hardness of about 83 Rc. Thesefixtures were used to mask selected portions of turbine vanes during airplasma spray of a 7YSZ workpiece coating thereon. These fixtures showedsignificant improvements over existing fixtures by substantiallyreducing the amount of overspray that occurred on the vanes, andvirtually eliminating the bridging of the workpiece coating between thevane and the fixture. This, in turn, substantially reduced the amount ofhandwork and rework that was required to prepare these vanes for use ina gas turbine engine. Furthermore, these fixtures were used forapproximately 250 cycles, and were still performing well. Thisrepresents a substantial improvement in useful life compared to existingfixtures.

As described above, this invention provides fixtures for use in maskingselected portions of workpieces so that workpiece coating is depositedonly on certain surfaces thereof. Advantageously, these fixturesminimize the amount of workpiece coating bridging that will occurbetween a fixture and a workpiece disposed therein. Additionally, thesefixtures minimize the amount of overspray that occurs on the workpiece.Therefore, the amount of handwork and/or rework that is necessary on thecoated workpieces is significantly reduced. This in turn improvesprocess yields and decreases cycle times. These fixtures are also muchmore durable than existing fixtures. All these advantages make thefixtures of this invention very desirable. Many other advantages will beapparent to those skilled in the relevant art.

Various embodiments of this invention have been described in fulfillmentof the various needs that the invention meets. It should be recognizedthat these embodiments are merely illustrative of the principles ofvarious embodiments of the present invention. Numerous modifications andadaptations thereof will be apparent to those skilled in the art withoutdeparting from the spirit and scope of the present invention. Forexample, while embodiments described herein relate to fixtures forselectively masking and coating turbine vanes, these fixtures may beeasily adapted for use in masking and coating many other gas turbineengine components (i.e., blade outer air seals, etc.) and/or componentsfor numerous other applications. This invention is not limited tofixtures for masking and coating gas turbine engine components. Thesefixtures may be used in any suitable coating process for the applicationof any workpiece coating that is primarily mechanically bonded to theworkpiece. Thus, it is intended that the present invention cover allsuitable modifications and variations as come within the scope of theappended claims and their equivalents.

1. A fixture for selectively masking portions of a gas turbine enginecomponent disposed therein while a workpiece coating is being applied tounmasked portions thereof, predetermined surfaces of the fixturecomprising: an average surface roughness of about 25 Ra or less tominimize the amount of workpiece coating bridging that will occurbetween the fixture and the gas turbine engine component when theworkpiece coating is applied; and a Rockwell hardness of about 65 Rc ormore to ensure the fixture can maintain an average surface roughness ofabout 25 Ra or less throughout repeated applications of the workpiececoating, where the predetermined surfaces include at least portions ofthe surfaces of the fixture proximate the gas turbine engine componentwhere workpiece coating bridging might occur across a gap in a plasmaspray coating proceess that is configured to minimize the amount ofrework needed on the gas turbine engine component after the gas turbineengine component is coated, and the fixture and the gas turbine enginecomponent disposed therein define the gap that is as small as about0.020 inches.
 2. The fixture of claim 1, wherein the predeterminedsurfaces comprise at least any fixture surfaces that will be exposed toworkpiece coating when the gas turbine engine component is being coated.3. The fixture of claim 1, wherein the fixture is capable of being usedin a thermal spray process.
 4. The fixture of claim 1, wherein thefixture is made of A2 tool steel at least partially coated with TiN orTiAIN.
 5. The fixture of claim 1, wherein the predetermined surfaces ofthe fixture comprise at least one of the following: (a) an uncoatedmaterial having a Rockwell hardness of at least about 65 Rc; (b) afixture coating on the surfaces, the coating comprising a Rockwellhardness of at least about 65 Rc; and (c) a treated material that hasbeen treated to have a Rockwell hardness of at least about 65 Rc.
 6. Thefixture of claim 5, wherein the predetermined surfaces have beenpolished to an average surface roughness of about 25 Ra or less.
 7. Thefixture of claim 5, wherein the fixture is made of at least one of: A2tool steel and 340 stainless steel.
 8. The fixture of claim 5, whereinthe fixture coating comprises at least one of TiN and TiAIN.
 9. Thefixture of claim 5, wherein the treated material has been treated via atleast one of a nitriding process and a plating process.
 10. The fixtureof claim 1, wherein the plasma spray coating process is a YSZ plasmaspray coating process.